In electrical power systems, damaging oscillations known as sub-synchronous resonance (SSR) oscillations occur when energy is interchanged between a torsional system of a turbine generator and a series-compensated transmission network via a generator air gap, at one or more natural frequencies of the combined system. Since the electric utility turbine generators of these systems may be seriously damaged through SSR oscillations, several approaches have been taken to compensate for and/or filter out these damaging oscillations.
One such approach is discussed in U.S. Pat. No. 5,367,197 "Series Capacitor Equipment" by Klerfors, which is hereby incorporated by reference. Klerfors discloses an "active-type" filter wherein a controllable circuit comprising an inductor and controllable semiconductor connection is connected in parallel to the series capacitor of a series-compensated transmission network. The filter transforms the energy of the subharmonic oscillation into fundamental frequency power and returns it to the power network. Some disadvantages in using this and other active-type filters with controllable semiconductor connections are as follows: many components are usually involved; the control device that is utilized may require sophisticated detection and control means; the thyristor switching process may create undesired harmonics that lower the quality of the power supply; and finally, the cost is generally high.
Another approach of filtering out undesirable SSR oscillations in transmission lines with series capacitors is through passive filters. Two examples of passive filters are disclosed in the following U.S. Patents which are hereby incorporated by reference. U.S. Pat. No. 5,262,677, "Reactor Subsynchronous Tuning Scheme" by Ramirez discloses a passive device in combination with a line inductive shunt reactor. At synchronous frequency the device acts as a short circuit, allowing the current of this frequency to pass through, while at SSR frequencies, the device attains a selected admittance magnitude through the capacitance and shunt reactor. The disadvantages of using shunt reactors in transmission lines, though, are that various and undesirable steady-state losses are associated with each shunt reactor, and usually shunt reactors are very costly.
U.S. Pat. No. 5,343,381, "Electrical Dampening Circuit for Dampening Resonance of a Power Signal in a Power Distribution Network" by Bolduc et al. discloses a parallel filter circuit for dampening resonance of a power signal. Bolduc uses a simple RCL circuit tuned to filter out a certain range of frequencies. One disadvantage of having a simple filter circuit such as the Bolduc filter is that it is limited to dampening at that certain range of sub-synchronous frequencies, and may not filter out all of the damaging frequencies. Furthermore, losses of power at power frequency may occur.
Thus, there is a need in the art to provide a passive parallel filter that will not only produce less power losses at power frequency but will provide dampening at a wider range of sub-synchronous frequencies.